Vince,
Please disable the "Show "Logging In" popup:" in the Configuration -> Login / Logout tab. Then, clear your Joomla caches (Help->Clear Cache).
There's a bug with that feature in Joomla 1.5 that will be fixed with the 4.2.1 release. The updated release should be out later today.
Sorry for the trouble, and if you still have issues after you disable that, just let us know, and we'll help you with any other problems.
Thanks,
Alex